The present invention relates to land and marine combustion turbines and in particular a spalling and corrosion resistant ceramic coating.
Marine and land based combustion turbines are subjected to different types of corrosion attack than aircraft turbine blades both because at least portions of their blades operate in a different (lower than aircraft) temperature region (and thus are also subjected to sulfide corrosion and intermediate temperature--1100.degree. to 1500.degree. F.--corrosion) and also are often run with less expensive fuels containing, for example, vanadates which can form highly corrosive molten salt deposits on the blade surfaces.
A variety of coatings have been used commercially in the past to minimize turbine blade corrosion. Such coatings include both diffusion coatings (e.g. platinum aluminide) and surface coatings (e.g. MCrAlY, where the M is a metal such as cobalt or nickel).
In addition, thermal barrier coatings have also been experimentally applied to turbine blade surfaces. Although such coatings also need to resist corrosion, such coatings need to be relatively thick as the primary function of those coatings is to provide insulation such that there is a temperature drop between the gas stream and a cooled metal blade. Such thermal barrier coatings typically have a dense ceramic outer coat, a porous ceramic (and thus insulating) intermediate layer and an MCrAlY bond coat on a nickel base superalloy blade. U.S. Pat. No. 4,255,495 issued to Levine et al. on Mar. 10, 1981 gives examples of thermal barrier coatings. Such coatings typically have a thickness of 20-30 mils to provide a thick enough porous ceramic layer to give a substantial (typically 100.degree.-200.degree. F.) temperature drop. Although this technique holds great promise, considerable difficulties have been incurred in such coatings and cracks in the coatings have resulted in spalling of the ceramic coating and also in trapping corrosive compounds such as molten salts against metal surfaces of the blade.
It has been discovered that a very thin (1-4 mils) coating of ceramic can, if applied hot (at a temperature of at least about 1200.degree. F.) is quite spalling and corrosion resistant. The coating of this invention is not intended to be a thermal barrier coating, and because of its lack of thickness, will not provide a substantial temperature drop. Like the thermal barrier coatings, a bond coat (preferably an MCrAlY) can be first applied on the blade surface. A porous ceramic is applied over the MCrAlY and a dense ceramic is then applied on top. Preferably both the porous ceramic and the dense ceramic are an yttria stabilized zirconia. It should be noted that the porous portion is a transition zone to allow for differences in thermal expansion and, being thin, provides little thermal insulation. It has been found that both keeping the coating very thin and controlling the temperature of the part to which the ceramic coating is applied to at least 1200.degree. F. are critical to producing a long lasting coating.